In diesel engines that directly inject fuel into the cylinders, the fuel was normally injected near the piston compression top dead center when there was high temperature and high pressure within the cylinder. In this case, ignition started and flames were formed while the fuel was being injected, and combustion continued by supplying subsequent fuel to the flames. Combustion of this form in which ignition starts during injection of the fuel in this way is normally known as diffuse combustion (hereafter also referred to as normal combustion), but this diffuse combustion has the problem that there is a limit to the reduction of NOx, smoke, and the like.
Therefore, in recent years, a form of combustion that is known as premixed compression ignition combustion (hereafter also referred to as premixed combustion) has been proposed to be realized in which the fuel injection or supply period is earlier than piston compression top dead center, and after fuel supply is completed, the premixed fuel-air mixture ignites (see Patent Documents 1, 2).
In premixed combustion, the premixed fuel-air mixture ignites after a certain period (premixed period) has passed after completion of fuel injection, so the premixed fuel-air mixture has sufficiently diluted and homogenized by the time ignition occurs. Therefore, the local combustion temperature is reduced, the quantity of NOx emissions is reduced, and combustion under insufficient air is avoided, so smoke generation is also reduced.
Patent Document 1: Japanese Patent Application Laid-open No. H11-324764
Patent Document 2: Japanese Patent Application Laid-open No. 2003-286880